High ethane recovery from a ryan/holmes additive recovery column

ABSTRACT

A method for separating a high ethane-content product from a high methane-content feed stream including the steps of introducing a high methane-content feed stream into a first distillation column containing a plurality of vapor-liquid contact devices and generating a first bottoms stream, and introducing the first bottoms stream into a second distillation column containing a plurality of vapor-liquid contact devices and generating a second bottoms stream. Additional steps include recovering an additive stream from the second bottoms stream and injecting the additive stream into the first distillation column, and recovering a high ethane-content product from the second distillation column.

PRIORITY CLAIM

[0001] This application claims the benefit of U.S. provisional patent application No. 60/317,093 filed on Sep. 4, 2001 the entirety of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention is in the field of cryogenic distillation.

BACKGROUND OF THE INVENTION

[0003] In the early eighties, the Ryan/Holmes process was developed for use in distillation processes. The Ryan/Holmes process uses the injection of an additive into a distillation column in order to increase the effectiveness and/or efficiency of the separation of a feed stream. The additive may be a by-product of the feed stream being processed or may be supplied separately. The Ryan/Holmes process is especially effective in cryogenic distillation processes, wherein certain components of the feed stream which would naturally solidify at the low separation temperatures, are maintained in a non-solid state.

[0004] The Ryan/Holmes process includes a stage wherein the additive, originally injected into a feed stream processing distillation column, is recovered for reuse. This stage may be a second distillation column. In the treatment of a high methane-content feed stream to generate a methane product stream, an additive of heavier hydrocarbons is used to prevent solidification of feed stream components. This additive may be recovered in a second distillation column. In this column, the process of removing undesirable components (typically CO₂ and/or H₂S) generates a “light” product stream containing these components as well as light hydrocarbons (typically C₂ and C₃ hydrocarbons).

[0005] Ethane product (C₂/C₃ light hydrocarbons), however, is desirable for use as an additive to the final methane product stream (to increase heating value) or as a product stream unto itself and so “loss” of ethane product into this “light” product stream is not desired. This invention provides a low cost method of obtaining a high ethane-content product stream (with few CO₂/H₂S components) from the additive recovery stage of a demethanization process. The invention also provides a method for purifying this high ethane-content product.

SUMMARY OF THE INVENTION

[0006] The invention involves a method for separating a high ethane-content product from a high methane-content feed stream. The steps in this process include, introducing a high methane-content feed stream into a first distillation column containing a plurality of vapor-liquid contact devices and generating a first bottoms stream, and introducing the first bottoms stream into a second distillation column containing a plurality of vapor-liquid contact devices and generating a second bottoms stream. Additional steps include recovering an additive stream from the second bottoms stream and injecting the additive stream into the first distillation column, and recovering a high ethane-content product from the second distillation column.

[0007] This is a low cost enhancement to a methane recovery system. It can be used to improve the heating value of final methane product from the methane recovery system. The process can also generate and additional revenue product stream. Such a process is advantageous for Alaskan North Slope gas processing where there is a limited market for recovered light hydrocarbons and, therefore, it is desirable to retain these components within the final methane product to enhance its heating value.

BRIEF DESCRIPTION OF THE FIGURES

[0008]FIG. 1 is a schematic flow diagram illustrating apparatus suitable for carrying out the invention described herein.

DETAILED DESCRIPTION OF PREFERRED AND ALTERNATE EMBODIMENTS

[0009] Introduction

[0010] A high methane-content feed stream may be processed to remove impurities and to generate a methane product. This process may take place at very low temperatures and utilize the Ryan/Holmes process. The separation takes place within a distillation column, having a plurality of vapor-liquid contact devices. The specifications of issued U.S. Pat. No. 4,462,814 (Holmes et al.), U.S. Pat. No. 4,318,723 (Holmes et al.), U.S. Pat. No. 4,350,511 (Holmes et al.), U.S. Pat. No 4,293,322 (Ryan et al.), U.S. Pat. No. 32,600 (reissue) (Ryan et al.) are incorporated herein by reference.

[0011] Ryan/Holmes Process

[0012] The Ryan/Holmes process utilizes the injection of an additive into a first distillation column to aid in the separation of components contained within the high methane-content feed gas stream. In a situation where the separation is performed under cryogenic conditions, the additive may prevent certain components contained within the feed gas stream from solidifying and plugging the distillation column. The additive used in the distillation may be externally added or may be one or more recycled components from the bottoms product taken from the first distillation column. When recycled components are used, a second distillation may be used to separate the additive from other products.

[0013] Demethanizer

[0014] In the process of the present invention, a first distillation column (Demethanizer) 100 is used to separate a high methane-content feed into an overhead product (methane product) which is substantially free of acid gas components (typically CO₂ and/or H₂S) and bottoms substantially free of methane. The high methane-content feed stream 10 generally contains C₁, C₂, C₃, C₄+ hydrocarbons, as well as N₂ and most often at least CO₂ and/or H₂S. The overhead product stream (methane product stream) 12 contains methane, N₂ and limited impurities: CO₂ (typically less than 3% by volume) and/or H₂S (typically less than 4 ppm). The bottoms stream 14 from the Demethanizer 100 contains the heavier hydrocarbons and larger concentrations of CO₂ and/or H₂S. An additive stream 16 recovered from a second distillation column (Additive Recovery Column) 120 is introduced into the Demethanizer 100 to maintain all components in a non-solid state. The additive stream 16 may contain C₂, C₃ and C₄+ hydrocarbons.

[0015] Additive & Ethane Recovery

[0016] The Additive Recovery Column 120 is used to separate the bottoms stream 14 from the Demethanizer into an overhead product stream 20 with high concentrations of CO₂, H₂S and light hydrocarbons and a heavy hydrocarbon bottoms stream 22. The heavy hydrocarbon bottoms removed from the Additive Recovery Column 120 supply the additive requirements for the Demethanizer 100. The heavy hydrocarbon bottoms stream 22 is split into the additive stream 16 and the net heavy hydrocarbon stream 24. A “high ethane-content” product side stream 18 may also be recovered from the Additive Recovery Column 120.

[0017] To facilitate the CO₂ and/or H₂S removal in the Demethanizer 100, the Additive Recovery Column 120 heavy hydrocarbon bottoms stream 22 is fractionated (as stated above) to be deficient in CO₂ and/or H₂S. Typically, this results in a substantial portion of the ethane within the bottoms stream 14 from the Demethanizer, to be “lost” within the CO₂ overhead stream 20. In the present invention, an ethane product is removed from the Additive Recovery Column 120, from a position which is not adjacent to the uppermost trays. In a preferred embodiment of the invention, the high ethane-content product stream 18 is taken from a position on the Additive Recovery Column 120 which is above a point “A” where the heavy hydrocarbon bottoms stream 16 is removed from the Additive Recovery Column 120 and below a point “B” where the Demethanizer's bottoms stream 14 is introduced into the Additive Recovery Column 120 second distillation column. The specific location between these points for the removal of the high ethane-content product stream 18 may be determined during operation so as to achieve the “optimum” combination of ethane recovery and additive separation. As a result of such processing, a product stream rich in high ethane is obtained. The high ethane-content product stream 18 will contain limited concentrations of CO₂ and/or H₂S.

[0018] The high ethane-content product stream 18 may be further treated to remove the limited amounts of CO₂ and/or H₂S. Removal may be performed in a variety of “standard” processing schemes, including, but not limited to: chemical reaction processes (such as amine treating), physical solvent processes (such as Rectisol—a commercially available physical acid gas removal process using an organic solvent such as methanol at subzero temperatures), hot carbonate processes, or batch processes (such as iron-sponge, caustic wash or solid potassium hydroxide bed).

[0019] Table 1 shows a typical material balance within the high ethane recovery process. (Demethanizer=DEM; Additive Recovery Column=ARC) TABLE 1 STREAM NUMBER <10> <12> <14> <16> STREAM DEM METHANE DEM DEM NAME FEED PRODUCT BOTTOMS ADDITIVE H2S LBMOLES/HR 1.1999 0.0058 1.2160 0.0220 CO2 ″ 1276.0981 128.4323 1147.6662 0.0003 N2 ″ 59.1030 59.1030 0.0000 0.0000 C1 ″ 7885.5616 7865.8477 19.7139 0.0000 C2 ″ 550.4845 1.5971 554.3099 5.4225 C3 ″ 192.3171 29.8115 706.0288 543.5231 C4+ ″ 35.2358 4.6422 410.2960 379.7024 TOTAL LBMOLES/HR 10000.0000 8089.4396 2839.2308 928.6704 TEMPERATURE ° F. 60 −110 81 −70 PRESSURE PSIA 650 615 630 625 H2S MOLE % 0.0120 0.0001 0.0428 0.0024 CO2 ″ 12.7610 1.5877 40.4217 0.0000 N2 ″ 0.5910 0.7306 0.0000 0.0000 C1 ″ 78.8556 97.2360 0.6943 0.0000 C2 ″ 5.5048 0.0197 19.5232 0.5839 C3 ″ 1.9232 0.3685 24.8669 58.5270 C4+ ″ 0.3524 0.0574 14.4510 40.8867 TOTAL MOLE % 100.0000 100.0000 100.0000 100.0000 H2S % OF FEED — 0.486 — — CO2 ″ — 10.064 — — N2 ″ — 100.000 — — C1 ″ — 99.750 — — C2 ″ — 0.290 — — C3 ″ — 15.501 — — C4+ % OF FEED — 13.175 — — STREAM NUMBER <18> <20> <22> <24> STREAM ETHANE CO2 ARC NET HEAVY NAME SIDE DRAW PRODUCT BOTTOMS HYDROCARBON H2S LBMOLES/HR 0.7278 0.4657 0.0226 0.0006 CO2 ″ 7.4754 1140.1904 0.0003 0.0000 N2 ″ 0.0000 0.0000 0.0000 0.0000 C1 ″ 0.0000 19.7139 0.0000 0.0000 C2 ″ 240.1650 308.5818 5.5630 0.1405 C3 ″ 141.0399 7.3817 557.6072 14.0840 C4+ ″ 20.7545 0.0001 389.5414 9.8390 TOTAL LBMOLES/HR 410.1628 1476.3335 952.7345 24.0641 TEMPERATURE ° F. 113 3 200 100 PRESSURE PSIA 373 365 375 350 H2S MOLE % 0.1774 0.0315 0.0024 0.0024 CO2 ″ 1.8226 77.2312 0.0000 0.0000 N2 ″ 0.0000 0.0000 0.0000 0.0000 C1 ″ 0.0000 1.3353 0.0000 0.0000 C2 ″ 58.5536 20.9019 0.5839 0.5839 C3 ″ 34.3863 0.5000 58.5270 58.5270 C4+ ″ 5.0601 0.0000 40.8867 40.8867 TOTAL MOLE % 100.0000 100.0000 100.0000 100.0000 H2S % OF FEED 60.658 38.809 — 0.047 CO2 ″ 0.586 89.350 — 0.000 N2 ″ 0.000 0.000 — 0.000 C1 ″ 0.000 0.250 — 0.000 C2 ″ 43.625 56.056 — 0.026 C3 ″ 73.337 3.838 — 7.323 C4+ % OF FEED 58.902 0.000 — 27.923

[0020] It may be readily appreciated that the present invention can be used in any number of applications without departing from the spirit or intent of the invention. While a preferred form of the invention has been shown in the drawings and the specification, since variations in the preferred form will be apparent to those skilled in the art, the invention should not be construed as limited to the specific form shown and described. 

1. A method for separating a high ethane-content product from a high methane-content feed stream comprising: a. introducing said high methane-content feed stream into a first distillation column containing a plurality of vapor-liquid contact devices and generating a first bottoms stream; b. introducing said first bottoms stream into a second distillation column containing a plurality of vapor-liquid contact devices and generating a second bottoms stream; c. recovering an additive stream from said second bottoms stream and injecting said additive stream into said first distillation column; and d. recovering a high ethane-content product from said second distillation column.
 2. The method of claim 1 wherein said high ethane-content product is recovered from a position on said second distillation column which is above a point where said second bottoms stream is removed from said second distillation column and below a point where said first bottoms stream is introduced to said second distillation column.
 3. The method of claim 1 wherein said additive stream comprises C₂, C₃ and C₄+ hydrocarbons.
 4. The method of claim 1 wherein said high ethane-content stream comprises predominantly C₂ and C₃ hydrocarbons.
 5. The method of claim 1 further including the step of removing a methane product stream from said first distillation column.
 6. The method of claim 5 wherein said high ethane-content product is blended with said methane product stream.
 7. A method for separating a high ethane-content product from a feed stream comprising: a. introducing said feed stream into a first distillation column containing a plurality of vapor-liquid contact devices and generating a first bottoms stream; b. introducing said first bottoms stream into a second distillation column containing a plurality of vapor-liquid contact devices and generating a second bottoms stream; c. recovering additive from said second bottoms stream and injection of said additive into said first distillation column; and d. recovering a high ethane-content product from said second distillation column, said high ethane-content product comprising: about 93 mole % C2 and C3 hydrocarbons with limited quantities of CO₂ or H₂S or a combination of CO₂ and H₂S. e. treating said high ethane-content product to remove CO₂ and H₂S.
 8. The method of claim 7 wherein the feedstream comprises C₁, C₂, C₃ and C₄+ components.
 9. The method of claim 1 further comprising the step of locating a high ethane-content product stream recovery point at a location optimizing ethane recovery and additive separation within said second distillation column.
 10. A method for separating a high ethane-content product from a feed stream including C₁, C₂, C₃ and C₄+ components, as well as CO₂ and/or H₂S comprising: a. introducing said feed stream into a first distillation column containing a plurality of vapor-liquid contact devices and generating a first bottoms stream; b. introducing said first bottoms stream into a second distillation column containing a plurality of vapor-liquid contact devices and generating a second bottoms stream; c. recovering an additive stream from said second bottoms stream and injecting said additive stream into said first distillation column; and d. recovering a high ethane-content product from said second distillation column.
 11. A high ethane-content product comprising C₂, C₃ and C₄+ hydrocarbons, said high ethane-content product manufactured by a process including the steps of: a. introducing a high methane-content feed stream into a first distillation column containing a plurality of vapor-liquid contact devices and generating a first bottoms stream; b. introducing said first bottoms stream into a second distillation column containing a plurality of vapor-liquid contact devices and generating a second bottoms stream; c. recovering an additive stream from said second bottoms stream and injecting said additive stream into said first distillation column; and d. recovering a high ethane-content product from said second distillation column. 